Field of the Invention
The present invention relates to a liquid ejection head configured to eject ink with a liquid ejection system to perform recording on a recording medium, and to a method of cleaning the liquid ejection head. Further, the present invention relates to a recording apparatus including the liquid ejection-head.
Description of the Related Art
A liquid ejection system (ink jet recording system) is configured to eject liquid (for example, ink) through ejection orifices provided in a liquid ejection head, and to cause the liquid to adhere onto a recording material such as paper, to thereby perform recording. The ink jet recording system, which is configured to eject liquid through use of bubbles formed in the liquid due to thermal energy generated by electrothermal conversion elements, is capable of achieving high image quality and high-speed recording.
This type of liquid ejection head generally includes a plurality of ejection orifices, flow paths communicating with the ejection orifices, and a plurality of electrothermal conversion elements configured to generate thermal energy to be used for ejecting ink. Each of the electrothermal conversion elements is formed of a heating element and an electrode configured to supply electric power to the heating element. The electrothermal conversion element is covered with a lower protective layer having an insulating property, which is formed of, for example, a silicon nitride, so that the insulating property is secured between the ink and the electrothermal conversion element.
During liquid ejection, a heat generation portion of the electrothermal conversion element is exposed to high temperature and subjected to a cavitation impact, which is caused along with formation and shrinkage of bubbles in the liquid, in conjunction with a chemical action, which is caused by the ink. Therefore, an upper protective layer is provided on the heat generation portion so as to protect the heating element from the impact caused by the cavitation and the chemical action caused by the ink. The temperature of the surface of the upper protective layer rises up to the vicinity of 700° C., and the surface is brought into contact with the ink. For those reasons, the upper protective layer is required to have film characteristics excellent in heat resistance, mechanical characteristics, chemical stability, and alkali resistance.
Further, coloring materials or additives contained in the ink are heated at high temperature and decomposed on the molecular level, thereby causing a phenomenon that those materials are transformed into insoluble substances called “kogation”. Physisorption of the kogation onto the upper protective layer causes non-uniform thermal conduction from the heating element to the ink, resulting in problems such as decrease in speed of ejected ink, instability of bubbles, and increase in energy required for ejection.
To address the problems, in Japanese Patent Application Laid-Open No. 2008-105364, there is disclosed a technology of removing the kogation by forming the surface of the upper protective layer with a material that is dissolvable through an electrochemical reaction, such as iridium or ruthenium.